1. Field of the Invention
This invention relates to method and apparatus for checking errors in a serial signal transmission system used in a centralized control system for various devices such as presses, machine tools, constructing machines, ships and airplanes as well as unattended conveyer systems and unattended warehouses.
2. Description of the Related Art
In order to control such devices as mentioned above in a centralized manner, a great number of sensors are required for sensing the states of each part of the device and a great number of actuators for-controlling the respective states of the device. The number of sensors and actuators is 3,000 or more in a typical press machine, and even more sensors and actuators are required in other devices. Other related apparatus are described in U.S. Pat. Nos. 5,105,426 and 5,170,338.
In a conventional system for controlling such devices in a centralized manner, there is provided a main controller to which a great number of sensors and actuators are connected. The main controller receives the outputs from the sensors and controls the actuators in accordance with signals from the sensors. When the number of sensors and the number of actuators are great, the number of lead cables connecting the main controller to the sensors and actuators are accordingly great and the structure of the input/output units of the main controller can be very complicated.
A construction has been proposed in which there are provided a plurality of nodes, each of which one or more sensors and actuators are connected to, and these nodes are connected to each other in ring or in series through the main controller such that each node is controlled by signals from the main controller. In such an arrangement, only a signal input line and a signal output line are required to be connected to the main controller and to each node. As a result, the number of lead cables is greatly reduced.
However, in the above-discussed construction, the problem arises how to ensure simultaneous input of the respective sensor outputs and simultaneous control of the respective actuators. To code with the problem, an arrangement has been proposed in which an address is allocated to each node to control each node in accordance with the address. However, there arises another problem of time lag caused by the address processing As a result, simultaneous input of the respective sensor outputs and simultaneous control of the respective actuators cannot be ensured satisfactorily.
In view of the foregoing, the inventors discarded the idea of allocating an address to each node connected to each other in series, and have proposed a series control apparatus in which each node is identified by the sequence (order) of the series connection, to thereby make address processing unnecessary, to eliminate a time lag due to the address processing and to greatly simplify the structure of the nodes.
According to the arrangement of the series control apparatus, each node sequentially adds the output signal of the sensor in the node to a signal from the upstream node in accordance with a predetermined rule, sequentially extracts a signal for the node from the upstream node in accordance with a predetermined rule, and outputs the extracted signal to an actuator in the node. In this apparatus, each node requires no address to thereby make address processing unnecessary. Time lag in each node is generated only during timing operation and therefore is reduced to a very small one, which greatly simplifies the structure of the nodes.
In the above arrangement, the main controller transmits a time series train of serial signals including an error check code such as horizontal and vertical parity check codes or a CRC (Cyclic Redundancy Check) code. Each node checks errors in the received signals by using the error check code. If there is an error involved, each node transmits a signal indicative of the error to the main controller through a downstream node, to thereby prevent a wrong operation of the system due to the transmission error.
However, in the conventional arrangements, when a plurality of error check codes such as horizontal and vertical parity check codes are added, signals resulting from error checking are collected together as a single signal and sent back to the main controller. Thus, the main controller cannot recognize the specific types of errors and therefore cannot cope with each type of error properly.